Nonlinear optical frequency conversion is commonly used to generate coherent optical radiation at wavelengths for which direct laser sources are inefficient or unavailable. For example, laser optical radiation (light) is often converted from the infrared spectrum, where efficient laser sources are available, into the visible spectrum, such as blue and green, since in general, efficient laser sources for the blue and/or green spectra are not available. Because nonlinear coefficients of transparent materials are small, several methods are used to enhance the efficiency of the interactions.
In general, second-order nonlinear effects are used most frequently in frequency conversion. Because second-order nonlinear effects use the lowest-order material nonlinearity, they exhibit the strongest nonlinear coupling coefficients. Examples of second-order nonlinear processes include second harmonic generation (SHG), sum frequency generation (SFG), difference frequency generation (DFG), and optical parametric generation (OPG), which includes optical parametric amplifiers (OPA) and optical parametric oscillators (OPO). In second-order nonlinear processes, the nonlinear coefficients of the nonlinear material couple three interacting optical fields (or waves), with differences between the above listed processes being the wavelengths of the interacting waves (which waves are the input waves, and which waves are the output waves).
In SHG, for example, the three interacting optical fields involve two photons at a fundamental input wavelength and one photon at an output wavelength (a second harmonic of the input wavelength, which is at twice the frequency of the fundamental frequency). SFG combines two strong lower-frequency fields to generate an output wave at a higher sum frequency, DFG generates a longer-wavelength difference wave from two strong input fields, while OPG generates two output wavelengths (a signal field and an idler field) from a single high-power input field (pump).
Generally, more efficient conversion is possible at higher optical intensities (i.e., higher power per unit area) and with longer interaction lengths within a nonlinear medium. Limits to frequency conversion may be set by available input power levels, material optical damage limits, reliability, nonuniformity or imperfection in the nonlinear materials (which may limit the effective interaction length), and so forth.
Commonly used techniques, such as placing a nonlinear material in its own optical resonator external to a source laser, may be used to enhance the power level of input radiation and effective interaction length. Alternatively, the nonlinear medium may be placed inside the cavity of a laser (e.g., an intracavity frequency converter) where there is high circulating fundamental intensity, and the converted light is taken as the output. Optical resonators and laser cavities may be of a standing-wave (bi-directional propagation) or ring (unidirectional propagation) type. Optical waveguides may also be used to increase interaction length over which light is kept at a high intensity. Standing-wave resonators are typically simpler to fabricate than ring resonators, but they generally have a bi-directional output requiring additional optics to combine the outputs. This results in unwanted complexity, as well as interference effects in the combined output beam, which may have detrimental effects on the combined output beam's optical quality and temporal stability.